The pesticides are playing a major role in the contamination of the environment. Thepresent investigation was done on physiological changes and deltamethrin responses of the freshwater ciliate Blepharisma intermedium exposed to a selective pyrethroid insecticide. The obtained LC50 values of Blepharisma intermedium was found to be 57.54ppm respectively. The acute toxicity tests were done by using probitanalysis and worked out sublethal concentrations were found to be 10ppm, 13ppm, 16ppm and 19ppm respectively. The contractile vacuole activity was reduced when the organisms were exposed to different concentrations in time dependent manner. The nuclear changes leading to DNA damage was noted using Feulgen fast green technique (Rizzo and Nooden;1973). The more abnormal forms were found to 67.2±0.79 percent at higher concentration 19ppm. The concentrations used for nuclear abnormality studies were 10ppm, 13ppm, 16ppm, 19ppm respectively.
